DESCRIPTION (Applicant's abstract reproduced verbatim): The long-range goal of the proposed studies is to understand the molecular genetic mechanisms controlling the establishment of motor neuron identities. The current study is focused on Drosophila as a model system but given the evolutionary conservation of these genetic programs, it is likely that our findings also will be of importance for the understanding of vertebrate motor neuron generation and differentiation. In the long term, these studies may provide novel insight into the nature and treatment of spinal cord diseases and injuries in humans. In this study we wish to address the function of two Drosophila members of the LIM homeodomain family of transcription factors, islet (isl) and lim3. Our studies show that these two genes are acting in a combinatorial code to specify motor neuron subtype identity. To address whether isl and lim3 act in the same or independent pathways for motor neuron specification, we now plan to study motor axon pathfinding in isl,lim3 double mutant lines. To address whether isl and lim3 can act together we plan to co-misexpress them using the GAL4/UAS system. The specific expression of isl and lim3 in subsets of Drosophila motor neurons raises intriguing questions about the regulation of these genes. We have previously identified their enhancers and now plan to dissect them hoping that this will lead to some insight into their regulation as well as provide valuable tools for future genetic screens. We further describe some of our preliminary work on the rotund gene, that we became interested in due to its genetic interaction with lim3. Our initial studies indicate that rotund is involved in motor axon pathfinding, but since the rotund cDNA has not been isolated the molecular mechanisms underlying this phenotype is unclear. To address these and other issues we plan to isolate and characterize the rotund gene.